1. Field of the Invention
The present invention relates to a multi-layered preform which is formed by performing a compression molding of a plural kinds of oriented thermoplastic resins and a multi-layered bottle which is formed by performing a biaxial stretch blow molding of the multi-layered preform, and more particularly to a multi-layered preform and a multi-layered bottle having layers each of which has no strain generated by the flow orientation at a bottom portion thereof and also exhibits the excellent environmental stress resistance (ESC property), the excellent impact resistance, the excellent strength and the excellent appearance characteristics of the bottom portion thereof.
2. Description of the Prior Art
Stretch blow molded bottles made of polyester, polypropylene, polyamide or the like, particularly polyester bottles exhibit the excellent transparency and the gas barrier property and hence, the stretch blow molded bottles have been widely used for containing a liquid detergent, a shampoo, a cosmetic, a seasoning such as a soy sauce, a sauce or the like, beer, cola, a carbonic acid beverage such as cider, a fruit juice, a beverage such as mineral water or the like.
On the other hand, with respect to the polyester bottle, for preventing the deterioration of a content due to oxygen from the outside, a multi-layered bottle which forms a gas barrier layer, an oxygen absorption layer or a gas barrier layer containing an oxygen absorption agent therein between an inner layer and an outer layer which are made of polyester layers is used for storing a content which is required to have the preservative property.
Further, for the purpose of recycling the polyester bottles, to recycle used bottles after collecting, multi-layered bottles are manufactured by making use of the recycled bottles as polyester layers which constitute outer layers or as polyester layers which constitute inner layers.
The polyester bottles are, in general, manufactured by performing the stretch blow molding of preforms which are formed by an injection molding. The manufacturing method is roughly classified into a hot parison method and a cold parison method. In the former hot parison method, a bottomed preform which is formed by an injection molding of polyester resin is subjected to the stretched blow molding in a softened state without being completely cooled. On the other hand, in the latter cold parison method, an overcooled bottomed preform having a size considerably smaller than a size of a final shape and having polyester formed in an amorphous state is preliminarily formed by performing the injection molding of polyester resin, then the preform is preheated to a stretching temperature and is stretched by being pulled in the axial direction in a mold for blow molding, and is subjected to the blow stretching in the circumferential direction.
With respect to the shape of the bottomed preform, the preform includes a mouth portion which corresponds to a mouth portion of the bottle and a bottomed cylindrical portion which is to be subjected to the stretched blow molding. To form a longitudinally elongated container, the preform which has a shape generally similar to a shape of a test tube is used. A thread portion which constitutes engaging means with which a sealing opening end or a lid is engaged is, for example, formed in the mouth portion. Further, at the center of the bottom portion, a gate portion which constitutes an inflow opening through which resin flows at the time of an injection molding or a trace of the gate portion is always formed.
In a single-layered preform manufactured by a conventional injection molding, at a gate portion of a bottom portion or in the vicinity of the gate portion, the turbulence of a resin flow is generated and the whitening is liable to occur. Accordingly, there has been a problem that a bottle obtained by performing the biaxial stretch blow molding of the preform exhibits the poor appearance characteristics such as whitening, the crazing or the like of the bottom portion and reduces the impact resistance.
With respect to the multi-layered preform manufactured by the injection molding and the bottle obtained by performing the biaxial stretch blow molding using such a multi-layered preform, since a plurality of resins are injected, the respective flows of the resins become more complicated and this tendency appears particularly noticeably.
Further, with respect to a multi-layered preform obtained by performing an injection molding of oriented thermoplastic resin such as polyethylene terephthalate or the like, even when a body portion is transparent, the cooling efficiency at a gate portion or at a periphery thereof is poor so that these portions are liable to easily suffer from the whitening. Such a phenomenon is called “whitening of gate” in general. It is said that such a phenomenon is generated due to a fact that the oriented thermoplastic resin which is subjected to residual strains caused by the flow orientation has a property to be extremely easily crystallized.
To prevent such whitening of gate, it is necessary to perform the injection molding under a condition that the residual strains caused by the flow orientation is not increased. For example, it has been known that the larger the diameter of the gate or the lower the injection pressure, the possibility of the occurrence of the whitening of gate is decreased.
However, there have been problems that when the diameter of the gate increased, the diameter of a gate residual portion at the bottom portion is increased and hence, the appearance characteristics are worsened and when the injection pressure is reduced, the profiling of the preform is worsened and, at the same time, the injection residence time is prolonged so that the productivity is lowered.
Further, the above-mentioned problems appear outstandingly with respect to the multi-layered bottle obtained by performing the biaxial stretch blow molding of the multi-layered preform. Particularly, at the time of performing the biaxial stretch blow molding, there has been a problem that an interlayer peeling-off is liable to be generated at a center portion or in the vicinity of the center portion of the bottom portion of the bottle so that the impact resistance is drastically deteriorated.
Inventors of the present invention have found that even when the residual strains caused by the flow orientations in respective layers which are generated at the time of performing the injection molding of the multi-layered preform are at a low level which does not bring about the above-mentioned whitening of gate, the multi-layered bottle obtained by performing the biaxial stretch blow molding of the multi-layered preform gives an adverse influence to the homogeneous and uniform stretch property, the crazing property, the environmental stress cracking resistance, the bottle strength and the like at the bottom portion thereof.
Accordingly, it is an object of the present invention to provide a multi-layered preform and a multi-layered bottle which are substantially free from the generation of residual strains caused by the flow orientation at respective layers of a bottom portion, are free from the occurrence of the interlayer peeling-off of respective layers of the bottom portion and ensure the homogeneous and uniform stretching whereby, as a result, the impact resistance and the buckling strength of the bottom portion are enhanced, the excellent environmental stress cracking property at the bottom portion can be obtained, the occurrence of the crazing or the whitening at the time of preservation can be eliminated, and the excellent appearance characteristics can be obtained.